Wow, that is boring. The martian Manhunter is a shapeshifter like Best-Boy from Teen Titans and yet you mostly find #34 stuff about them doing plain, dull, uninspired sex. If I had shapeshifting powers then I would transform myself into Cthulhu and have sex with 13 women at once! C'mon guys, if I want plain, dull, uninspired sex then I watch the generic shit you found everywhere on the interwebs.

I can't cite my sources, but one explanation I remember was Superman invented special lamp for over his bed that imitated red sun radiation enough to basically reduce him to the level of a normal human man while it was on. Admittedly, probably not a great deal of spontaneity in their lovemaking, and one can only listen to that "Turn on the Red Light" song play so many times before the thrill is gone. :P